Printed circuit board assemblies and methods of making same are described in the art, with examples being defined in U.S. Pat. Nos. 3,969,177, 4,515,878, 4,521,280, 4,554,405, 4,662,963, 4,700,016 and 4,705,592, all of which are assigned to the assignee of the present invention. Attention is also directed to U.S. Pat. Nos. 3,617,613, 3,922,386, 3,958,317, 4,093,768, 4,191,800, 4,400,438, 4,420,509, 4,446,173, 4,456,657, 4,496,793, 4,522,667, 4,587,161 and 4,689,270 wherein various board structures and techniques for making same are described.
As defined herein, the present invention describes a circuit member (e.g., a power plane) and a process for making same wherein the member eventually forms part of a circuit board assembly of the multilayered variety. Typically, such assemblies as used in computer products include a plurality of signal planes used in combination with associated power planes located at designated levels in the composite structure depending on the product's operational requirements. As stated, assemblies of this type are described in several of the aformentioned patents.
One such method of making a power plane (or core) member involves the steps of laminating several such power planes (each including a central layer of prepreg, a known material comprised of a thermosetting resin impregnated glass fiber having conductive coatings, e.g., copper, on opposing sides thereof) and thereafter processing this laminated "stack" as a singular unit. The prepreg in this instance is provided to add strength during processing but does not electrically isolate the two outer conductive planes. In effect, each resulting power plane thus comprises two individual layers of the conductive copper. These planes, as stated, are laminated and subjected to subsequent operations (e.g., milling, drilling) after which these are separated. Disadvantages of this method include, among others: (1) the entire power plane is electrically active all the way to its outer edges which in turn necessitates "staggered" design power planes of different sizes and patterns if these are to be successfully used in multilayered end products; (2) electrical design is limited to only one voltage level per plane; (3) any damage (e.g., burrs, indentations) at the periphery of such power planes may prove detrimental to the final board (notably, the presence of copper on the outer surfaces of the supporting prepreg promotes burr formation during subsequent drilling); (4) use of such a stacked unit during processing necessitates use of at least two "sacrificial" planes per unit, which two planes must be disposed of following subsequent machining operations; and (5) the requirement of using an interim prepreg layer for support in processing and in the final power plane unit results in additional, undesired thickness to the multilayered board having a plurality of such planes therein.
Another such method of manufacturing power planes involves what is also referred to as the subtractive method. Again, a prepreg layer is used having copper coated on one or both sides thereof. This member is etched (e.g., with a solution of cupric chloride and hydrochloric acid), a photoresist is applied, exposed and developed (using e.g., a solution of sodium carbonate as the developer), the developed member then being etched (e.g., cupric chloride solution) and stripped (e.g., using sodium hydroxide). While this process was able to produce a power plane having electrical isolation therein, it possessed several disadvantages, including particularly the requirement for wet/chemical processing and the several disadvantages associated therewith (e.g., need for complex and elaborate equipment). Additionally, etched holes have proven to be susceptible to residual copper, residual chemical contamination and may also include remnants of the fiber glass cloth and other debris therein. Such holes are thus subject to failure (e.g., by electrical shorting between such holes as plated and other conductors within the composite board).
The power plane produced in accordance with the teachings of the instant invention uniquely provides, among other features, electrical isolation in at least two regions thereof (and several more if desired), can be produced in a facile manner adaptable to mass production, and can be made without wet/chemical processing. In summary, the power plane as defined herein is produced using mechanical procedures which are relatively easy to perform and significantly advantageous in comparison to prior techniques.
It is believed that such a power plane, and a method of making same, would constitute a significant advancement in the art.